tests: check ETag format in test-hgweb-commands

hgweb: emit a valid, weak ETag Previously, ETag headers from hgweb weren't correctly formed, because rfc2616 (section 14, header definitions) requires double quotes around the content of the header. str(web.mtime) didn't do that. Additionally, strong ETags signify that the resource representations are byte-for-byte identical. That is, they can be reconstructed from byte ranges if client so wishes. Considering ETags for all hgweb pages is just mtime of 00changelog.i and doesn't consider of e.g. .hg/hgrc with description, contact and other fields, it's clearly shouldn't be strong. The W/ prefix marks it as weak, which still allows caching the whole served file/page, but doesn't allow byte-range requests.

policy: add cffi policy for PyPy This adds cffi policy in the case where we don't want to use C modules, but instead we're happy to rely on cffi (bundled with pypy)